This invention relates to methods and apparatus for providing dispersion compensation in high-speed optical transmission networks and systems, and in particular to chromatic dispersion and polarisation mode dispersion compensation, including high-order polarisation mode dispersion.
Fibre-optic transmission systems are now being developed for tens of gigabits-per-second (Gbit/s) communication channels, whilst large volumes of 10 Gbit/s systems are being fully deployed into existing networks. Various potential limits are approached as the performance of such transmission system is pushed further. The phenomenon of polarisation mode dispersion, PMD, is a problem recently attracting a great deal of attention from the telecommunications industry. PMD is a type of distortion that varies from fibre to fibre and is typically of greater magnitude in older fibres. PMD is also a random phenomenon, varying with both time and optical frequency. Whilst service providers are reluctant to invest in new fibre routes, PMD may restrict the deployment of new systems over the older fibre routes of their network. In a small number of fibres, PMD will give rise to distortions so large that a 10 Gbit/s optical transmission system cannot be reliably deployed over the route. The impact of PMD scales linearly with system bit-rate, hence PMD will become a greater problem as the bit-rate of systems are increased. It is for these reasons that PMD solutions have to be found.
PMD is a fundamental characteristic of both optical fibres and optical components. It arises from the consideration that single mode fibre can actually support two weakly guided modes that are orthogonally polarised. In other words, given an ideal fibre, a pulse can be launched into either of these two polarisation modes and propagate through the fibre in that polarisation mode alone. A fiber exhibits slightly different refractive indices along different axes, a physical characteristic known as birefringence, Birefringence arises from a variety of intrinsic and extrinsic features of the fibre manufacture. These features include geometric stress caused by a noncircular core, and stress birefringence caused by unsymmetrical stress of the core. Other sources of birefringence include external manipulation of the fibre. External forces will include squeezing the fibre, bending the fibre and twisting of the fibre.
In a birefringent fibre, the propagation speed will vary with the launch polarisation gate into the polarisation modes of the fibre. Consequently, when proportions of the pulse are launched into both polarisation axes they travel at different speeds and hence arrive at different times. The magnitude of the difference in arrival times between the fastest and slowest paths through the fibre is known as the differential group delay (DGD).
The receiver of a direct detection optical transmission system does not distinguish between the different polarisation modes, but simply detects the combination of the different polarisation modes. The difference in arrival times of the pulse through the two polarisation modes will degrade the quality of the received data.
In a long length of fibre the birefringence is expected to be weak but vary randomly along its entire length. This leads to random mode coupling along the fibre, a process by which the pulse will continuously couple power between the two polarisation modes of the fibre. The phenomenon of PMD relates to the random variation of the DGD of the fibre. The DGD is expected to vary randomly over time due to random variations of the fibre birefringence as a result of environmental effects, such as temperature. A consequence of this random variation means that the instantaneous DGD of a fibre cannot be predicted. Instead the DGD of a fibre must be described statistically. The fibre DGD also varies over frequency/wavelength.
The DGD is the first-order consideration of PMD. It makes the assumption that the PMD characteristics of a fibre are constant over the bandwidth of the transmitted data signal. Higher-orders of PMD are considered when the PMD characteristics can no longer be considered constant over the bandwidth of a signal. Higher-order PMD relates to the variation of the PMD characteristics of a fibre with frequency.
In order to compensate for first order PMD, it has been proposed to use a delay line which provides differential delay for different polarisation states, in order to reverse the system fiber DGD. A particular class of fibres, known as high birefringence (Hi-Bi) fibres, has been engineered deliberately to have very high, uniform birefringence for this purpose. The fibres have two clearly definable axes with different refractive indices. The propagation speed of a pulse will differ greatly between each axis.
Three categories of techniques are used for PMD compensations. They are all-optical, all electrical, and hybrid.
For all-optical PMD compensation, the restoration of PMD distortion is done optically without any optical-electrical conversion. The signal remains in the optical domain. Normally, all-optical PMD compensators consist of a polarization controller and either a variable or a fixed birefringent delay element, such as a piece of high birefringence optical fiber. The basic concept is to find the principal states of polarization (PSP) of the fiber into the axes of the birefringent delay element to reverse the DGD of the system fiber. A control feedback signal is used to give an indication of the level of distortion imparted on the transmitted data at the receiver, after the PMD compensator. This can then be used to adjust the alignment of the polarization controller, and the variable delay element (if being used) to provide maximum compensation of the PMD distortions to the data. Filtered components of the received electrical spectrum can be used to give an indication of the quality of the received data.
In the all-electrical method, the distorted optical signal is converted to an electrical signal at the receiver. A delay line filter with specific weights is used to partially compensate for the distortion due to PMD.
Hybrid PMD compensation is a technique that uses both optical and electrical methods to restore the distortion due to PMD. For example a polarization controller (PC) and a polarization beam splitter (PBS) can be used to transform the states of polarization, and split the polarization components. At each output of the PBS, a high-speed photo-detector converts tile optical signal to electrical signal. An electrical delay line is used to adjust the phase delay between the two electrical signals.
Although there are various techniques for compensating for first order PMD, higher data rates now result in the higher-order effects of PMD becoming significant. A detailed analysis of the properties of higher order PMD will not be given, but a brief analysis follows.
The PMD can be represented as a three-dimensional dispersion vector, {right arrow over (xcexa9)}. The magnitude of the vector represents the instantaneous DGD, xcex94xcfx84, of the fibre.
xcex94xcfx84=|{overscore (xcexa9)}|
The dispersion vector will often be defined directly in terms of the DGD and the position of the fat PSP of the fibre, {circumflex over (q)}. The dispersion vector is defined as follows:
{right arrow over (xcexa9)}=xcex94xcfx84{circumflex over (q)}
In the first order, PMD impact is simply dependent upon the instantaneous DGD and on the angle of orientation of the PSPs, or more specifically the relative angle between the PSP orientation and the launch angle of light into the fiber.
The form of the second-order PMD characteristics can be considered by taking the first derivative of the PMD dispersion vector with respect to optical frequency.             Ω      _        ω    =                    ⅆ                  Ω          →                            ⅆ        ω              =                                                      ⅆ              Δ                        ⁢                          xe2x80x83                        ⁢            τ                                ⅆ            ω                          ⁢                  q          ^                    +              Δ        ⁢                  xe2x80x83                ⁢        τ        ⁢                  xe2x80x83                ⁢                              ⅆ                          q              ^                                            ⅆ            ω                              
Second-order PMD can thus be divided into two components; the linear dependence of DGD with frequency (the first term), and the linear rotation of the positions of the PSP""s of a fibre with frequency (the second term).
According to the invention, there is provided a polarization mode dispersion (PMD) compensation arrangement for receiving an optical input data signal which has been subjected to PMD, the arrangement comprising an adaptive chromatic dispersion compensator and a first-order PMD compensator, wherein the adaptive chromatic dispersion compensator is controlled to provide compensation for both chromatic dispersion and second order PMD).
The invention is based on the realisation that second order PMD may be considered equivalent to chromatic dispersion. In other words, second-order PMD results in deformations of a transmitted pulse that are identical in nature to those attributed to chromatic dispersion. Therefore, second-order PMD can be considered as an additional polarization dependent chromatic dispersion term, which results in a linear variation of the propagation speed of the spectral components of a transmitted pulse.
The arrangement of the invention thus uses a chromatic dispersion compensator to provide second-order PMD compensation, based on the realisation outlined above. A (conventional) first order PMD compensator provides first order PMD compensation, so the total system provides chromatic dispersion compensation as well as first and second order PMD compensation.
Preferably, the adaptive chromatic dispersion compensator and the first-order PMD compensator are in series.
Preferably, a feedback loop from the output of the arrangement is used to derive control signals for controlling the adaptive chromatic dispersion compensator and the first-order PMD compensator, This feedback loop may provide electrical spectrum analysis of the output.
As one example, the first-order PMD compensator may comprise a polarization controller and a fixed birefringent delay element. The adaptive chromatic dispersion compensator may comprise first and second chirped Bragg reflection gratings, wherein at least the first grating is coupled with a strain applicator for applying axial strain to the grating for turning the grating. The strain applicator may comprises a piezoelectric transducer on which the chirped grating is mounted.
Preferably, the piezoelectric transducer can be tuned so that the grating provides a selected level of dispersion compensation within less than 1 ms.
The strain applicator of the first grating may enable dispersion compensation within a range of less than 60 ps/nm, and wherein the second grating is also coupled with a strain applicator which enables dispersion compensation within a range greater than 60 ps/mm. The second order PMD results in dispersion deviations of less than 60 ps/nm, and this is required to respond rapidly, whereas compensation over the larger range is for normal chromatic dispersion compensation, which is able to respond more slowly.
The invention also provides a method of providing polarization mode dispersion (PMD) compensation comprising:
using an adaptive chromatic dispersion compensator to compensate for chromatic dispersion and second-order PMD; and
using a first-order PMD compensator to compensate for first order PMD.
The compensation arrangement is preferably implemented in a node for an optical communications system.